This invention relates to hysteresis circuits and, more particularly, to hysteresis circuits which are free from source voltage fluctuations.
FM stereo tuners generally include a stereo indicator circuit for indicating the stereo reception state and a commonly termed FM muting circuit for muting noise between channel and channel when tuning. These circuits are varieties of the level discrimination (switching) circuit and are realized by Schmitt trigger circuits having hysteresis characteristics.
As is well known in the art, such a Schmitt circuit includes a first and a second transistor with their emitters commonly connected. These transistors are connected together with their collector resistors and common emitter resistor between a power supply and a reference potential point.
Therefore, the hysteresis voltage of this circuit, which is determined by an "on" level of the input voltage that is required for turning on the first transistor and turning off the second transistor and an "off" level of the input voltage required for turning off the first transistor and turning on the second transistor, is directly influenced by the source voltage fluctuations.
This means that the hysteresis characteristic of the circuit is adversely affected by the source voltage fluctuations and is also considerably influenced by drift.
Where a Schmitt trigger circuit which is subject to the aforementioned various influences is used as a level discrimination (switching) circuit in the stereo indicator circuit or FM muting circuit mentioned above, therefore, malfunction is inevitable, which is a grave drawback.
U.S. Pat. No. 4,259,601 and No. 3,700,921 disclose hysteresis circuits. These hysteresis circuits, however, also have similar drawbacks as in the Schmitt trigger circuit since their hysteresis characteristics have dependency upon the source voltage fluctuations.